1. Field of the Invention
The present invention relates to spark plugs.
2. Description of the Related Art
A typical spark plug for use in an internal-combustion engine includes a tubular metal shell, a tubular insulator disposed in an inner hole of the metal shell, a central electrode disposed in a tip-end-side axial hole of the insulator, a metal terminal disposed in a rear axial hole, and a ground electrode having two ends, one of which is joined to a tip-end side of the metal shell and the other one of which is disposed so as to face the central electrode to form a spark discharge gap therebetween. In the axial hole of the insulator and between the central electrode and the metal terminal, a glass seal portion (also referred to as an “electrically-conductive seal portion”) containing a resistive material for noise reduction is disposed.
A typical glass seal portion is formed by compressing, by hot press, a glass-seal-use powder material containing resistive powder and glass powder. For performing hot press, the central electrode, the glass-seal-use powder material, and the metal terminal are sequentially inserted into the axial hole of the insulator and compressed by hot press, so that the glass-seal-use powder material is consolidated. This consolidation enhances the properties (load life characteristics) of the resistive material of the glass seal portion. In addition, the glass seal portion is fixedly attached to the central electrode and the metal terminal and exerts its effect of impact resistance (shock absorption) or airtightness.
With the development of internal-combustion engines in recent years, reduction of the diameter of a spark plug has been demanded. The reduction of the diameter of a spark plug involves reduction of the outer diameter of the insulator. This reduction of the outer diameter causes a problem of rendering the insulator easily breakable and various other problems described below.
First, a case is assumed where the inner diameter of the axial hole of the insulator is retained at the same level as the inner diameter of an axial hole of an existing insulator despite the reduction of the outer diameter of the insulator. In this case, the insulator is thinned in accordance with the amount of reduction of the outer diameter of the insulator. Thus, the voltage resistance characteristics are more likely to decrease and the insulator is rendered more easily breakable as a result of failing to endure the inner pressure that occurs during a hot press operation. This configuration, however, is advantageous in that it allows the use of metal terminals having an outer diameter the same as that of existing metal terminals since the inner diameter of the axial hole of the insulator remains unchanged from that of an existing insulator and that the load the same as that of the case of an existing spark plug can be transmitted to the glass-seal-use powder material during a hot press operation.
Second, a case is assumed where the diameter of the axial hole of the insulator is reduced in order to secure the thickness of the insulator after the reduction of the outer diameter of the insulator. In this case, a metal terminal inserted in the axial hole of the insulator is also accordingly thinned. When the metal terminal is thinned, the metal terminal may be bent as a result of being yielded by the load that occurs during a hot press operation. When the metal terminal is bent, the load that occurs during a hot press operation fails to be fully transmitted to the glass-seal-use powder material, whereby properties (for example, shock resistance or resistive material properties) may become insufficient. This configuration, however, allows the insulator to have a thickness the same as the thickness of an existing insulator. Thus, the insulator is advantageous in that it has the same voltage resistance characteristics and it is less easily breakable during a hot press operation.
To address the above-described problems in the first and second cases, a technology is known in which a tapered portion is disposed in the glass seal portion (Japanese Unexamined Patent Application Publication No. 2009-541916). In Japanese Unexamined Patent Application Publication No. 2009-541916, a tapered portion is provided inside a large-diameter portion (flange portion) of the insulator. In the tapered portion, the outer diameter of the glass seal portion (that is, the axial hole diameter of the insulator) decreases toward the tip end of the spark plug. This configuration enables an increase of the axial hole diameter of the insulator at a portion into which the metal terminal is inserted while the thickness of the insulator at a middle body portion disposed closer to the tip end than the large-diameter portion of the insulator is secured. Thus, the metal terminal can retain a large outer diameter. As a result, the metal terminal is rendered less easily bendable also during a hot press operation, so that insufficient compression of the glass-seal-use powder material due to bending of the metal terminal is less likely to occur.